According to the agreed way forward for Rel-10 feedback [1], a precoder for a subband is composed of two matrices, one of which targets wideband and/or long-term channel properties, denoted by W1, and the other targets frequency-selective and/or short-term channel properties, denoted by W2. The eNB determines the W1 via long-term wideband feedback, and determines the W2 via short-term narrowband feedback.
For closely-spaced multi-antenna, the precoding scheme could take the antenna-related characteristic brought by the closely spacing into consideration, so as to simplify the design of the precoding scheme. The industry has discussed about the precoding codebook of the closely-spaced cross-polarized CLA. In R1-103026, Samsung has proposed their two-stage feedback approach with W1 a M×r matrix and W2 a M×M square matrix; and in R1-101742, Ericsson has proposed their two-stage feedback approach with W1 a M×2 matrix and W2 a 2×r matrix, wherein M is the amount of the antennas and r is the amount of the data flows. The performance of both above approaches under the MU-MIMO scenario, such as data throughput, is not satisfying.